This invention relates to a process for producing a ferrofluid, and a composition thereof. More particularly, fine particles of the ferromagnetic materials such as magnetite, ferrite, iron, cobalt alloy, etc. are dispersed stably in one dispersing medium selected out of an oil group, an ester group or an ether group, whereby a ferrofluid composition having a high magnetizing capacity is produced effectively.
Generally, the ferrofluid is a colloidal solution, in which such ferromagnetic fine particles are dispersed stably and uniformly in a preferred dispersing medium. Such colloidal solution is neither coalesced nor precipitated under the influence of magnetic force, gravity, centrifugal force, etc., so that the ferromagnetic fine particles are not separated from the colloidal solution. Thus, the ferrofluid displays a strong magnetic force responsive to magnetic field.
In recent years, such ferrofluid has been used as a sealing agent, a damping agent, a lubricant or the like and various industrial circles pay high attention to its unique properties.
Various dispersing mediums can be used for the ferrofluid. When it is used for a lubricant or a sealing agent in bearing means, it must have good lubricating property, high heat resistance, low volatility, good chemical stability, etc. From this point of view, the oil group such as a mineral oil, a synthetic oil, etc., the ester group and the ether group are most suitable as dispersing mediums for such ferrofluid. In this case, each surface of the ferromagnetic fine particles is required to have a lipophilic nature to be well-adapted to the dispersing medium.
When the ferrofluid is used as a sealing agent, the stronger magnetic force thereof causes the stronger sealing force. Further, when it is used as a lubricant, the stronger magnetic force thereof is capable of coping with the mechanical agitation caused by an axial rotation of a rotary shaft, thereby the ferrofluid is prevented from splashing or spoiling the surrounding.
The strength of magnetization is dependent upon the concentration of the ferromagnetic fine particles contained in the ferrofluid. Accordingly, it is a very important task to obtain the ferrofluid having a higher concentration thereof. However, if the concentration is higher and higher, a gap between adjacent particles becomes slighter and coalesced easily. Accordingly, a highly concentrated ferrofluid cannot be prepared without realizing the optimum dispersion of the ferromagnetic fine particles in a desired dispersing medium.
If many fine particles of large diameter that are coalesced easily are contained in the dispersing medium or if a surface-active agent is adsorbed insufficiently on each surface of the fine particles, it becomes impossible to obtain a highly concentrated ferrofluid.
We will now refer to the conventional process for producing a ferrofluid, which is disclosed in Japanese Unexamined Patent Publication No. 44579/.[.1967.]. .Iadd.1976.Iaddend.. This process also uses any one out of the oil group, the ester group or the ether group as a dispersing medium. This conventional technique may be called a dispersion method using two molecular adsorption layers.
First of all, an aqueous suspension of colloidal ferromagnetic oxide is obtained by the wet method. According to the wet method, alkali is added to an acid solution including ferrous ion and ferric ion respectively at the ratio of 1:2, and the thus obtained mixture has more than about pH 9. The mixture is matured at a suitable temperature, so that a magnetite colloidal solution is obtained. To make lipophilic each surface of the collaidal particles obtained thus, the surface-active agent containing an unsaturated fatty acid or its salt as a main material is added to the solution. Then, an excessive quantity of the surface-active agent is added to the solution in order to seal completely each surface of the colloidal particles, thereby two molecular adsorption layers are formed. It is generally known that a first monomolecular adsorption layer of the surface-active agent ion displays a lipophilic nature on its surface, but a second monomolecular adsorption layer displays a hydrophilic nature on its surface. Under those circumstances, it is difficult to separate the liquid phase from the solid phase. Then, by adjusting pH of the solution, the ferromagnetic fine particles are rapidly coalesced and settled, thereby separation of the solid from the liquid phase becomes difficult. Subsequently, by filtering and purifying a sediment, the surface-active agent ions of the second molecular layer are removed and the surfaces of the fine particles become lipophilic. Then, they are treated with dehydration and drying. Finally, those dried fine particles are dispersed in a desired dispersing medium.
Although such a dispersing method is advantageous in the point that the hydrophilic colloidal particles having a difficult filterability can be coalesced rapidly by adjusting pH, it has the following disadvantages:
(1) The ferromagnetic particles becomes dispersoid of the obtained ferrofluid, but there are many larger particles in diameter. Accordingly, when those ferromagnetic particles are dispersed in a preferred dispersing medium, the larger particles are precipitated, so that it is not possible to obtain a highly-concentrated ferrofluid having a high magnetizing capacity.
(2) Because the surface-active agent is added excessively more than the quantity necessary for forming a monomolecular layer, an oily unsaturated fatty acid is formed and adsorbs the surfaces of the particles.
Finally, the particles adsorbed by the unsaturated fatty acid are removed by a later process. Accordingly, this conventional method is inefficient.
(3) The surface-active agent is added to the aqueous suspension having an alkali pH and more than an equipotential point of the colloidal particles, under which condition the surfaces of the colloidal particles are of a negative charge. Thus, for example, the surface-active agent such as unsaturated fatty acid having a negative charge in the aqueous solution is unsusceptible to adsorb the particles. Accordingly, some unstable particles are susceptible to arise in the dispersing medium.